It has been widely practiced to react a polyol compound and a polyisocyanate compound in the presence of a blowing agent, etc. to produce a rigid polyurethane foam or a rigid polyurethane urea foam (which may be hereinafter generally referred to as a rigid foam). Foamed states of such rigid foams are generally classified into two categories of open-cells and closed-cells. In the open-cells, individual foamed cells are communicated with one another, and a rigid foam having many open-cells is excellent in the dimensional stability and is used for civil engineering work or as a thermal insulating material for buildings or building materials.
Various compounds are known as blowing agents to be used for producing rigid open-cell foams. Heretofore, as a blowing agent, a low boiling point fluorinated compound has been mainly used. However, such a low boiling point fluorinated compound has a problem of accelerating the global warming, if it is present in the atmosphere. Therefore, a technique has been proposed to use water as a blowing agent as much as possible thereby to reduce the amount of the fluorinated compound to be used (e.g. Patent Documents 1 to 3).
In a case where water is used as a blowing agent so much, the storage stability of the mixed liquid (hereinafter referred to as the polyol system liquid) prepared by mixing the raw material polyol compound with water, catalyst, etc. tends to deteriorate. Accordingly, when it is foamed, the foamed state of the foam is likely to be non-uniform. Especially, in the case of a spraying method, depending upon the site to be sprayed, independence of foamed cells, the cell size, etc. tend to be different, or partial shrinkage or depression of the foam is likely to occur, thus leading to adverse effects to the physical properties or the appearance.
Further, in the application to buildings or building materials, reduction of the weight of rigid foams is desired. As a means to reduce the weight, it is desired to use a blowing agent as much as possible. However, as the amount of water to be used as a blowing agent increases, the above-mentioned problem of deterioration in the storage stability of the polyol system liquid, a problem of deterioration in the compatibility between the polyol system liquid and the liquid containing a polyisocyanate compound (which may also be referred to as the polyisocyanate liquid), or a problem of mixing deficiency, is likely to appear more distinctly. That is, it is very difficult to accomplish the weight reduction while maintaining the physical properties and appearance. Especially in a spraying method for on-site foaming (a practical method for reacting the polyol system liquid and the polyisocyanate liquid while spraying them onto a substrate surface) wherein it is required to increase the reactivity, it has been difficult to constantly produce a rigid foam which is light in weight and has open-cells. Further, in thermal insulation work, not only horizontal or downward spraying to walls or floors, but also upward spraying to ceiling is required. In a spraying method, the reactive raw material-mixed liquid is reacted after sprayed in a liquid state on a substrate. If the speed until the liquid mist undergoes foaming increases, there will be a problem that the foam will hang down like ice pillars.
Patent Documents 1 and 2 propose a method for producing a rigid open-cell foam by foaming with water by using a polyol mixture of a low molecular weight polyol and a high molecular weight polyol. However, the amount of water used here is relatively small, and no consideration is made about the problem with respect to the storage stability of the polyol system liquid attributable to the formulation of the water blowing agent.
Patent Document 3 discloses a method for producing a low density rigid open-cell polyurethane foam by foaming with water by means of slab foaming. This method is a method for producing a rigid open-cell polyurethane foam by using water and using a polyether polyol (a) having an average number of functionality of from 2.5 to 4 and a hydroxy value of from 200 to 300 mgKOH/g, a polyether polyol (b) having an average number of functionality of from 4 to 6 and a hydroxy value of from 400 to 900 mgKOH/g and a polyether polyol (c) having an average number of functionality of from 2.5 to 3.5 and a hydroxy value of from 20 to 60 mgKOH/g in combination.
Patent Document 3 discloses a production method by means of a slab-foaming process, but discloses nothing about a spraying method wherein a high reactivity is required. No consideration is made also with respect to the problems of the surface property and sagging of the foam in a case where a spraying method is applied to a house, and no process is disclosed which makes it possible to form a uniform thermal insulation layer. In the urethane-foaming method such as a slab-foaming or injection method, it is possible to freely determine the mixing ratio of the polyol system liquid to the polyisocyanate liquid, but in the case of a spraying method, usually, there is a restriction such that the mixing ratio is about 1:1 by volume ratio. In a case where water is used in a large amount for the purpose of weight reduction, foaming is carried out under such a restriction and therefore, formulation for e.g. a slab-foaming process is not useful as reference. Further, no consideration is made about the storage stability of the system liquid. The slab-foaming process is carried out usually in a building, and the polyol system liquid is used up in a short period of time. Therefore, no consideration is required for the storage stability of the liquid. On the other hand, the spraying method is carried out outdoors in many cases, and the polyol system liquid is stored under various conditions and therefore is required not to undergo separation for at least about one month.